Photonics grating coupler and method of manufacture

ABSTRACT

A structure for coupling an optical signal between an integrated circuit photonic structure and an external optical fiber is disclosed as in a method of formation. The coupling structure is sloped relative to a horizontal surface of the photonic structure such that light entering or leaving the photonic structure is substantially normal to its upper surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/669,937, filed Oct. 31, 2019, now U.S. Pat. No. 11,041,990; which isa continuation of U.S. application Ser. No. 16/138,159, filed Sep. 21,2018, now U.S. Pat. No. 10,473,861; which is a continuation of U.S.application Ser. No. 15/664,975, filed Jul. 31, 2017, now U.S. Pat. No.10,209,449; which is a continuation of Ser. No. 14/976,677, filed Dec.21, 2015, now U.S. Pat. No. 9,753,226; which is a continuation of U.S.application Ser. No. 13/829,893, filed Mar. 14, 2013, now U.S. Pat. No.9,239,432; each of which is incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with Government support under Agreement9999404-12-0008 awarded by DARPA. The Government has certain rights inthe invention.

FIELD OF THE INVENTION

Embodiments of the invention provide a structure for coupling opticalsignals between an integrated circuit photonics device, e.g., awaveguide, and an external optical fiber.

BACKGROUND OF THE INVENTION

Optical signal transmission may be used to communicate signals betweenseparated integrated circuit chips to provide inter-chip connections andwithin components on the same integrated circuit chip to provideinter-chip connections. In many instances, it is necessary to couple anexternal optical fiber to a photonic device, e.g., a waveguide, of anintegrated circuit photonics chip. Such coupling requires preciseoptical alignment between the optical fiber and the photonic device tomaximize optical signal transmission between them.

However, coupling light into and out of a sub-micron integrated photonicdevice, such as a waveguide, with high efficiency is difficult becauseof the small waveguide mode size compared with that of an optical fiber.

Such optical coupling is made conventionally through a diffractiongrating coupler provided on a planar upper surface of a waveguide whichchanges the direction of an optical signal passing through the waveguidefrom being generally parallel to the running length of waveguide to adirection which is out of the waveguide.

FIG. 1 illustrates in cross section an example of a prior art gratingcoupler. An integrated circuit photonic structure 10 is provided whichhas a silicon-on-insulator (SOI) substrate having a silicon base 11, aburied oxide (BOX) 13, typically formed of silicon dioxide, formed oversilicon base 11, and a silicon fabrication material 26, which is formedinto a waveguide core 15. The BOX 13 provides a lower cladding for thesilicon waveguide core 15 and a further oxide material 17, which has aflat upper surface 22, is provided as side and an upper cladding for thewaveguide core 15. A grating coupler 21 is formed in the upper surface18 of the waveguide core 15 to direct light passing between thewaveguide core 15 and an optical fiber 131. The optical fiber 131 has acore 133 and outer cladding 135.

As shown, light entering into or exiting from the grating coupler 21 inthe direction of arrows A is angled along optical axis B relative to theupper surface 22 of upper cladding 17. This angling of light along axisB is an inherent characteristic of grating coupler 21. Depending on thedesign of the grating coupler 21, including materials used, the opticalaxis B is at an angle in the range of about 8 to about 12 degrees from adirection normal to the upper surface 22 of photonic structure 10. As aresult, if an optical fiber 131 is arranged to be normal to the uppersurface of the photonic structure there is a considerable optical signalpower loss, as much as 50%, between the grating coupler 21 and opticalfiber 131. Thus, to obtain maximum efficiency in the transfer of lightbetween the grating coupler 21 and optical fiber 131, the optical fiber131 must, as shown, also be angled by a like amount relative to theupper surface 22 of the photonic structure 10. This complicatespackaging of the photonic structure 10 as a mechanical angled couplingmust be provided for the optical fiber 131. Moreover, the connectionbetween the angled optical fiber 131 and photonic structure 10 typicallyrequires an active alignment system to ensure alignment of the opticalfiber 131 to the photonic structure 10 along optical axis B. This addscosts and complexity to the packaging of the photonic structure 10.

What is needed is a grating coupler and method of formation whichprovides an optical signal which is emitted to or received by an opticalgrating coupler in a direction substantially normal to the upper surface22 of the photonic structure 10 to facilitate mechanical coupling withan optical fiber 131.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in cross-section a prior art arrangement of aphotonic structure with an optical fiber:

FIG. 2 illustrates in a cross-section an embodiment of the invention;

FIGS. 3A-3I illustrate in cross-section one embodiment of a method forforming the FIG. 2 embodiment; and,

FIG. 4 illustrates in cross-section the coupling of the FIG. 2embodiment with an external optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments described herein provide a grating coupler for an integratedphotonic structure, and a method of its formation, which achieves alight coupling into and out of the photonic structure in a directionwhich is substantially normal to an upper surface of the photonicstructure. In the context of this application substantially normalencompasses normal and a deviation of no more than 2 degrees fromnormal. As a result, assembly of the integrated circuit with an externallight fiber can be facilitated, without requiring an active alignmentstructure between the two.

FIG. 2 illustrates in cross-section one embodiment of an integratedphotonic structure 100 which has an optical axis B between a gratingcoupler 109 and an external fiber (FIG. 4 ) in a direction substantiallynormal to an upper surface 120 of the photonic structure 100. Itcomprises a substrate 101, for example a silicon substrate, and a lowercladding material 103 of an oxide, for example, silicon dioxide. Thelower cladding material 103 is provided with a generally horizontalportion having a horizontal upper surface 104 and an upwardly slopingportion having an upwardly sloping upper surface 105. The photonicstructure 100 further comprises a waveguide core 107 formed of, forexample, silicon provided over the lower cladding material 103. Thewaveguide core 107 has a horizontal portion 106 and an upwardly slopingportion 108. The silicon used for waveguide core 107 can bepolycrystalline silicon, single crystal silicon or amorphous silicon.The slope of upper surface 105 of lower cladding material 103 is at anangle C within the range of about 8 degrees and about 12 degreesrelative to the horizontal upper surface 104 of the lower claddingmaterial 103, and also relative to an upper surface of horizontalportion 106 of waveguide core 107, and also relative to an upper surface120 of photonic structure 100. The exact angle C is dependent on thedesign of grating coupler 109 and materials used, as described below.

An upper cladding material 111, formed of for example, an oxide, e.g.,silicon dioxide, or of silicon nitride, is provided over and around thesides of waveguide core 107. The upper cladding material 111 has aseries of grooves 113 therein over the upwardly sloping portion 108 ofthe waveguide core 107. The grooves 113 extend into an upper surface of,but not through, the upwardly sloping portion 108 of the waveguide core107 to form a sloped grating coupler 109. The grating coupler 109 isalso sloped by the angle C.

The photonic structure 100 further comprises an oxide material 115, forexample, silicon dioxide formed over an upper surface of upper claddingmaterial 111 which extends into grooves 113 to complete, with claddingmaterial 111 and oxide material 103, a surrounding cladding for thewaveguide core 107 and sloped grating coupler 109. In one specificexample, the slope angle C can be in the range of about 11.5 to about 12degrees, the depth of the grooves in the upper surface of the slopedportion 108 of the waveguide core 107 may be in the range of about 270nm to about 280 nm and the period of the grooves may be about 498 nm,although other slope angles C within the range of about 8 degrees toabout 12 degrees can be used. Other depths and periods can also be useddepending on the design of the grating coupler 109 and materials used.

Light transmitted along the horizontal portion 106 of waveguide 107(i.e., along optical axis A) passes into the sloped grating coupler 109which directs the light to exit an upper surface 120 of photonicstructure 100 in a direction of optical axis B which is substantiallynormal to the upper surface 120. Likewise, light entering into thephotonic structure 100 at a location over grating 109 in a direction ofoptical axis B, will be directed by the grating 109 into the horizontalportion 106 of waveguide 107 along axis A. Thus, light enters or leavesthe photonic structure 100 at an angle which is substantially normal toupper surface 120, which minimizes optical signal power loss andfacilitates assembly with an optical fiber 131 in the manner describedbelow.

One manner in which the FIG. 2 structure can be fabricated is nowdescribed with reference to the cross-section views in FIGS. 3A through3I.

FIG. 3A illustrates a substrate 101 having a flat upper surface andwhich may be of any suitable material for supporting an integratedphotonic structure, including semiconductor materials such as silicon. Alower cladding material 103 is provided over substrate 101 and,depending on the material used for waveguide core 107, has an index ofrefraction lower than that of the waveguide core 107 material. If thewaveguide core material is silicon, the lower cladding material 103 maybe an oxide, for example, silicon dioxide. The lower cladding material103 may be deposited by any known deposition technique, for example byPECVD or may be grown if the substrate 101 is a silicon substrate. Thethickness of the deposited lower cladding material 103 may be in therange of about 1.5 μm to about 3.0 μm. The upper surface of the lowercladding material may be planarized, e.g., by CMP, to provide a flatsurface which is parallel to the flat upper surface of substrate 101 tofacilitate further fabrication.

FIG. 3B illustrates the deposition of a photoresist material 117 overthe lower cladding material 103. The deposited photoresist material 117may be planarized, e.g., by CMP, such that the upper surface is flat.The deposited photoresist material 117 is then patterned using a knowngray scale lithography technique for example, using high resolution 193nm laser light and a graduated gray scale mask, to create the slopedupper surface 116 in the resist material 117 shown in FIG. 3C. Areactive ion dry etch (RIE), or a deep reactive ion dry etch (DRIE) isperformed on the photoresist material 117 illustrated in FIG. 3C whichconsumes the photoresist material 117 and transfers the pattern of theupper surface of photoresist material 117 into the upper surface oflower cladding material 103, as shown in FIG. 3D. After etching, thelower cladding material 103 has a horizontal upper surface 104 and anupwardly extending sloped upper surface 105. The thickness t1 of thelower cladding material 103 at the horizontal upper surface 104 can bein a range of from about 0.5 um to about 1 um and the thickness t2 ofthe lower cladding material 103, at the end of the sloped upper surface105 can be in the range of about 1 um to about 2 um.

FIG. 3E illustrates deposition of a waveguide core 107 over the uppersurface of the lower cladding material 103. Waveguide core 107 may beformed of any suitable material for forming an optical waveguide,including silicon and may have a uniform thickness. Any suitable knowndeposition technique can be used to form waveguide core 107 including,PEVCD and sputtering, among others. As illustrated in FIG. 3E, thewaveguide core 107 has a horizontal portion 106 and an upwardly slopedportion 108 which corresponds to the sloped upper surface 105 of thelower cladding material 103. The waveguide material 107 is initiallydeposited as a blanket layer which is then masked and etched to theupper surface of the lower cladding material 103 to form waveguide core107. FIG. 3F shows a ninety degree rotated cross sectional view of theFIG. 3E structure along the lines 3F-3F and the resultant waveguide core107 after the masking and etching of the waveguide core blanketmaterial.

Following formation of waveguide core 107, and as illustrated in FIG.3G, an upper cladding material 111, which may be an anti-reflectivecoating, is then deposited over the waveguide core material 107. Thedeposited upper cladding material 111 is planarized to have a flatsurface which is substantially parallel to the flat surface of substrate101 and the flat upper surface of the horizontal portion 106 ofwaveguide core 107. Any suitable deposition technique can be used todeposit material 111 including PECVD. The upper cladding material 111has an index of refraction lower than that of the waveguide core 107material. For a silicon waveguide core 107 the anti-reflective coatingwhich provides the upper cladding material 111 can be silicon nitride,or an oxide such as silicon dioxide.

Next, as illustrated in FIG. 3H, the upper cladding material 111 isetched to form a plurality of parallel grooves 113 therein which extendnot only through the upper cladding material 111 but also into an uppersurface of the sloped portion 108 of the waveguide core 107. The grooves113 are vertically oriented relative to an upper surface of thehorizontal portion 106 of waveguide core 107. The plurality of grooves113 extend into the upper surface of sloped portion 108 of the waveguidecore 107 and form grating coupler 109. The etching of grooves 113 may beperformed by reactive ion etching, or other wet or dry etchingtechnique. The bottom of the grooves 113 have a profile which matchesthe slope profile of the upper surface of sloped portion 108. In oneexample, the grooves may extend into the upper surface of sloped portion108 of the waveguide core 107 by an amount in the range of about 270 nmto about 280 nm. The grooves 113 may extend the entire width of thewaveguide core 107. In one example, the grooves 113 can also be spacedon a pitch of about 498 nm. However, other groove depths and periods,including non-uniform periods, can be used depending on the desireddesign of the grating coupler 109, and materials used.

Next, as shown in FIG. 3I, additional cladding material such as an oxidematerial 115, for example, silicon dioxide, is deposited to overcoatupper cladding material 111 and to fill in the grooves 113. The uppersurface 120 of oxide material 115 can be planarized such that it isparallel to the horizontal upper surface of 104 of the lower claddingmaterial 103. By filling in the grooves 113 with the oxide material 115,the upper cladding for waveguide core 107 is complete such that asurrounding cladding for waveguide core 107 and grating coupler 109 isprovided by lower cladding material 103, upper cladding material 111,and grooves 113 filled with oxide material 115. As an option, the oxidematerial 115 can also be planarized to the upper surface of the uppercladding material 111 after grooves 113 are filled in which case theupper surface of the upper cladding material 111 provides the uppersurface of the photonic structure 100.

Grating coupler 109 formed on the sloped portion 108 of waveguide core107 provides a direction change for light passing into or out ofwaveguide core 107 and into or out of the photonic structure 100illustrated in FIG. 3I. The angle with which light enters or leaves thephotonic structure 100 is at a direction substantially normal to anupper surface 120 of the photonic structure 100. The angle is alsosubstantially normal to materials in the photonic structure 100including the horizontal portion of waveguide core 107, and the uppersurface of upper cladding material 111. As noted, depending on thedesign of the grating coupler 109 and materials used, the slope angle Cfor grating 109 is in the range of about 8 degrees to about 12 degreesrelative to upper surface 120. Thus, the direction of light into or outof the grating coupler 109 is such that the angle between the directionof light paths A in the horizontal portion 106 of the waveguide core 107and B into and out of the grating coupler (FIG. 2 ) is substantiallynormal, that is, at 90 degrees.

The precise angle C will be different for different physicalcharacteristics of the grating coupler 109, including materials used andlocation and spacing of the grooves 113, and materials used for thesurrounding cladding. Accordingly, the exact slope angle for aparticular grating coupler 109 within the range of about 8 degrees toabout 12 degrees relative to the upper surface 120 can be determined inadvance. One technique for determining the slope angle of a specificgrating coupler 109 in advance is to first fabricate a horizontalgrating coupler of the same materials and which has the same groovestructure as a grating coupler 109 to be fabricated. The exit angle oflight propagating through the horizontal grating coupler is measured fordeviation from a direction normal to the upper surface of the waveguidecore. This deviation angle is then used as the slope angle C in thegrating coupler 109 fabricated as described above with reference toFIGS. 3A through 3I. In a second more preferred technique, theconstruction of the planar waveguide grating coupler is simulated by acomputer and the deviation from normal of the light emitted by thesimulated planar waveguide is determined and then used to set the slopeangle C of the grating coupler 109 fabricated as described withreference to FIGS. 3A through 3I. For the grating coupler 109 having thematerials, groove depth and groove pitch as described above, a slopeangle of about 11.5 to about 12 degrees has been found suitable toproduce an optical direction B of entry or exit of light in thewavelength range of 1525 nm to 1565 nm or 1180 nm to 1260 nm into orfrom the grating coupler 109, which is substantially normal relative toupper surface 120. In general, the grating period A follows theequation:

A = n eff - n top ⁢ Sine ⁢ θwherein n_(eff) is the effective refractive index of the waveguide core,n_(top) the refractive index of the cladding,

_(O) is the free space wavelength of light passing through a waveguide,and θ is the emitting angle of light in a standard non slopped gratingcoupler. θ is the angle which is needed for the slope angle C to achievea substantially normal emission from the sloped grating coupler 109. Anychange in etch depth, grating period, the duty cycle and slope, orcombination of them, will change n_(eff) and affect the wavelength

_(O) passing through the waveguide.

FIG. 4 shows the addition of a fiber alignment structure 119 to theupper surface of oxide material 115. The alignment structure can befabricated of any suitable material, for example, silicon dioxide and isprovided such that an optical fiber 131 held by the alignment structurehas an end face which sits squarely over the grating coupler 109. Forexample, the alignment structure 119 can be fabricated as a collar forsurrounding an optical fiber 131. Since the angle of light entering orleaving the integrated circuit structure illustrated in FIG. 4 issubstantially normal to the upper surface 120 of the photonic structure100, the optical fiber 131 can be easily fixed to the photonic structure100 in a direction substantially normal to upper surface 120 by anoptically transparent adhesive 123. Thus, a simplified method forproviding a photonic structure 100 which can be easily packaged forconnection with an external optical fiber 131 and which does not need anactive alignment structure is provided.

The various identified materials can be varied, as can the structure ofthe grating coupler 109 which is formed, either in the depth of thegrooves 113 or period of the grooves in order to accommodate specificwavelengths of light traveling through waveguide 107 and into or out ofthe photonic structure 100. Also, while waveguide core 107 is describedas being formed of silicon, which may be polycrystalline silicon, singlecrystalline silicon, or amorphous silicon, other materials known to besuitable for forming a waveguide core can also be used. Such othermaterial include silicon nitride (Si₃N₄), silicon oxynitride(SiO_(x)N₄), silicon carbide (SiC), silicon germanium (Si_(x)Ge_(y)),gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), indiumgallium arsenide (InGaAs), indium phosphor (InP), or other lighttransmission materials. In this list of materials x and y represent apositive integer. Likewise other materials than those discussed abovecan be used for cladding materials 103, 111, 115, as long as the indexof refraction of the material of the waveguide core 107 is higher thanthat of the surrounding upper and lower cladding materials 103, 111,115.

While embodiments of an optical grating coupler 109 and method of itsformation have been described and illustrated, the invention is notlimited by these embodiments. Also, while the photonic structure 100 isshown as having an upper surface 120 of a cladding material 115, itshould be apparent that additional light transmissive materials can beformed over the cladding material 115 as part of photonics integratedcircuit. Also, as described, cladding material 115 can be planarizeddown to the upper surface of cladding material 111 such that the uppersurface of cladding material 111 provides the upper surface of photonicstructure 100. In addition, while the sloped grating coupler 109 isdescribed as being provided in a waveguide core 107, it may also beprovided in a light path of other photonic devices.

Accordingly, the invention as described above with reference to specificembodiments is not limited by the foregoing description but is onlylimited by the scope of the appended claims.

What is claimed is:
 1. An integrated photonic structure having a planarupper surface, the integrated photonic structure comprising: asubstrate; a waveguide disposed over the substrate; the waveguideincluding a first region with a first lower surface parallel to theplanar upper surface and a second region with a second lower surfacesloped at an acute angle to the planar upper surface, the second regionincluding a sloped grating coupler configured to redirect light into orout of the waveguide core in a direction normal to the planar uppersurface of the integrated photonic structure; and a collar disposed overthe planar upper surface of the integrated photonic structure andvertically aligned with the sloped grating coupler, the collarconfigured to surround a planar end of an optical fiber and verticallyalign the optical fiber with the sloped grating coupler.
 2. Theintegrated photonic structure of claim 1, wherein the sloped gratingcoupler comprises spaced grooves in a sloped portion of the waveguide.3. The integrated photonic structure of claim 1, wherein the slopedgrating coupler has a slope angle configured to redirect the lightthrough a redirection angle of about 90 degrees.
 4. The integratedphotonic structure of claim 1, further comprising the optical fiber anda layer of adhesive attaching the planar end of the optical fiber to theupper planar surface of the integrated photonic structure within thecollar.
 5. An integrated photonic structure having a planar uppersurface, the integrated photonic structure comprising: a substrate; alower cladding material disposed over the substrate; a waveguide coredisposed over the lower cladding; and an upper cladding materialdisposed over the waveguide core, the upper cladding material includinga first region with a first lower surface parallel to the planar uppersurface and a second region with a second lower surface sloped at anacute angle to the planar upper surface, the second region including asloped grating coupler; a collar disposed over the planar upper surfaceof the integrated photonic structure and vertically aligned with thesloped grating coupler; and an optical fiber having a planar end adheredto the planar upper surface of the integrated photonic structure withinthe collar, wherein the sloped grating coupler is configured to redirectlight between the waveguide core and the optical fiber through aredirection angle of about 90 degrees.